Titanium dioxide can be prepared via the "sulfate" process or by the hydrolysis of titanium tetrachloride. In the latter process, titanium tetrachloride is added to water to form a titanium hydrolysate. The hydrolysate is then calcined to form the finished titanium dioxide. For example, in U.S. Pat. No. 2,426,788, a titanium tetrachloride solution is added at a controlled rate to a small amount of hot water maintained at or near the boiling point. The titanium tetrachloride is hydrolyzed and precipitates out as a hydrolysate, which is then calcined. In U.S. Pat. No. 2,337,215, a titanium oxychloride nucleating material is used to initiate the hydrolysis of titanium salt solutions.
In U.S. Pat. Nos. 3,329,484 and 3,528,773, pigment-grade titanium dioxides are disclosed. In each of these patents, a master solution is prepared by digesting titanium containing ore in a large excess (at least 100%) of hydrochloride acid resulting in a high chloride to titanium ratio solution. U.S. Pat. No. 3,329,484 discloses the precipitation of titanium dioxide from the master solution at temperatures of about 100.degree. C. and higher and at autogenous pressure. The precipitated titanium dioxide may or may not be calcined to further develop the tinting strength. U.S. Pat. No. 3,528,773 discloses a two-step process wherein the first step consists of the precipitation of titanium dixoide from the master chloride solution at temperatures from about 100.degree. C. and higher and at autogenous pressure, and the second step consists of "re-treating" the precipitated titanium dioxide with an acidic solution so as to produce titanium dioxide of increased crystal size. The increased crystal size also results in increased tinting strength such that calcination is not needed. Both Nos. 3,329,484 and 3,528,773 disclose processes which produce rutile crystals.
Titanium dioxide with anatase crystal structure is normally prepared by the sulfate process. This process involves multiple steps starting with digestion of titaniferous ores in sulfuric acid which gives a solution containing essentially all of the impurities which were present in the ore. Next, the solution is hydrolyzed and precipitated. The precipitation step leaves much of the impurity content in solution. Thus, the resulting hydrolysate must still be subjected to additional purification steps, such as by filtering and washing. Lastly, the hydrolysate is calcined to obtain the final product, which is highly suited to its intended use, but it does not possess the very high purity which is often desired for some of the new high technology applications.
To enhance anatase production, up to 0.75% K.sub.2 CO.sub.3 (based on TiO.sub.2 equivalent pulp content) may used as a conditioning agent in the sulfate process. Additionally, some impurities, such as phosphorous which is inherent from the ore, cannot be separated by any known conventional means, generally leaving up to 0.75% P.sub.2 O.sub.5 in normally prepared anatase TiO.sub.2.